1. Field of the Invention
The invention relates to a process for aluminizing steel in which a steel is dipped in a liquid bath containing aluminum.
2. Discussion of the Background
When a dipping process is used to provide an aluminum layer on steel, the coating which is obtained on the steel generally is stratified into several layers. These include:
an inner layer in contact with the steel, composed of one or more alloys of aluminum from the bath and iron from the steel. It also is referred to as an alloyed layer; and PA1 an outer layer, generally thicker, comprising an aluminum-based main phase. PA1 1. dipping the steel casting in the bath at a temperature as low as possible to limit the growth of the interfacial alloy layer; PA1 2. using a liquid aluminizing bath whose composition corresponds, at liquid-solid equilibrium, to the area of existence of the .tau..sub.6 or .tau..sub.5 solid phases. PA1 the composition and mean temperature of the bath are adjusted to be in equilibrium with the phase designated as .tau..sub.5 or the phase designated as .tau..sub.6, preferably with the .tau..sub.6 phase. PA1 this liquid bath is saturated with iron. PA1 the immersion temperature of the steel is higher than the bath temperature. PA1 if the silicon content in the bath is approximately 8%, the immersion temperature ranges between about 700 and about 740.degree. C., preferably about 720.degree. C. PA1 if the silicon content in the bath is approximately 9%, the immersion temperature ranges between about 720 and about 765.degree. C., preferably about 730.degree. C. PA1 if the silicon content in the bath is approximately 9.5%, the immersion temperature ranges between about 740 and about 760.degree. C., preferably about 740.degree. C.
Since the inner alloy layer tends to be brittle in nature, steps are generally taken to limit thickness. These include the addition of materials to the dipping baths to inhibit alloying between aluminum and steel. Silicon is the most widely used alloying inhibitor. Its weight concentration in the dipping bath generally ranges between 3 and 13%.
In continuous aluminizing processes, the dipping baths are saturated with iron due to a partial dissolution of the steel in the bath. This saturation is known to lead to the formation of mattes and the liquid bath is in equilibrium with the solid phase of these mattes.
Under the usual conditions of aluminizing, the two previously cited layers which form the aluminized coating may be more precisely described as follows. The alloyed interfacial layer is composed essentially of a phase designated as .tau.5 and/or a phase designated as .tau.6. According to the conditions of aluminizing, this layer may be subdivided into several alloyed
sub-layers, particularly in the case of the present invention. The outer layer is composed principally of aluminum in the form of broad dendrites. These dendrites are saturated with iron and, as the case may be, with silicon in solid solution.
The .tau.5 phase has a hexagonal structure and crystallizes in the form of globular grains; it sometimes is referred to as .alpha..sub.H or H. The iron content of this phase generally ranges between about 29 and about 36% by weight; the silicon content of this phase generally ranges between about 6 and about 12% by weight; the balance is composed principally of aluminum. The chemical composition corresponds approximately to the formula Fe.sub.3 Si.sub.2 A.sub.12.
The .tau.6 phase has a monoclinic structure and crystallizes in the form of elongated, flat grains; it sometimes is referred to as .beta. or M. The iron content of this phase generally ranges between about 26 and about 29% by weight; the silicon content of this phase generally ranges between about 13 and about 16% by weight; the balance is composed principally of aluminum. The chemical composition corresponds approximately to the formula Fe.sub.2 Si.sub.2 Al.sub.9.
FIG. 1 is a three-dimensional representation of an Al--Si--Fe ternary phase diagram, where the variations--vertical axis--of the temperature of equilibrium of a liquid phase with different solid phases are designated as follows: FeAl.sub.3.ident..THETA., Fe.sub.3 Si.sub.2 Al.sub.12.ident..tau..sub.5, Fe.sub.2 Si.sub.2 Al.sub.9 .ident..tau..sub.6, FeSiAl.sub.3, .ident..tau..sub.2, FeSi.sub.2 Al.sub.4.ident..delta., Al.ident.aluminum, Si.ident.silicon, and other phases such as .tau..sub.3.tau..sub.4.
The .theta. phase plays a significant role in the present invention. Its structure is monoclinic and it may contain up to about 6% by weight of silicon in solid solution; the chemical composition therefore corresponds approximately to the formula FeAl.sub.3.
In FIG. 1, Si=0% and Fe=0% which means Al=100%. This Figure makes it possible to establish the nature of the solid phases which are capable of being in equilibrium with an aluminizing bath in the liquid state, in terms of the composition of the bath, and the temperature of the bath at equilibrium.
FIG. 2 is a projection of FIG. 1; the liquid-solid equilibrium temperature is determined with the aid of isothermal curves. The temperature interval between each curve is 20.degree. C.
Table 1 summarizes the possible composition of the .theta., .tau.5 and .tau.6 phases.
TABLE 1 Composition of the bath and of the main pbases obtained after solidification of the aluminum coating Composition Weight % Al Si Fe Bath &gt;86 3 to 13% Saturation (ex.: 3%) Eutectic 87 12.2 0.8 .tau.6 Phase 55 to 61 13 to 16 26 to 29 .tau.5 Phase 55 to 62 6 to 12 29 to 36 .theta. Phase 52 to 64 0 to 6% 36 to 40
An Al--Si--Fe eutectic with a melting temperature of 578.degree. C. is shown in Table 1.
As indicated above, the inner interfacial layer of the aluminum-based coating tends to be brittle and has a tendency to crack at the time of shaping of the aluminized castings. This cracking results in a decrease in the corrosion protection provided by the coating. To obtain coatings which are more resistant to cracking during shaping and to corrosion, it is desirable to limit the thickness of this interfacial layer.
According to the prior art, in order to achieve this purpose, the following two conditions should be maintained:
Condition 2 leads to the use of baths with silicon contents in excess of 7.5%, and preferably 9% (see FIG. 1 and 2).
According to document EP 0 760 399 (NISSHIN STEEL) and document JP 4 176 854-A (NIPPON STEEL), in a continuous process for aluminizing a steel strip, it is recommended that the strip be immersed at a temperature below the mean temperature of the bath. Thus, for a bath containing 9% silicon with the temperature generally ranging between 650 and 680.degree. C., the immersion temperature of the strip should be at a maximum of 640.degree. C.